Luminescent ceramic converters typically are used in white-light-emitting pc-LEDs to convert a portion of the blue light emitted from an InGaN LED semiconductor die (or chip) into a yellow light. The remaining unconverted blue light that passes through the converter and the yellow light emitted by the converter combine to generate an overall white light emission from the pc-LED. The luminescent ceramic converter in a pc-LED device is typically a thin, flat plate of a dense, luminescent ceramic that is affixed to the surface of the LED chip so that the plate is in close proximity to the light emitting surface. For white light generation, the material of the converter is usually based on cerium-activated yttrium aluminum garnet (Y3Al5O12), also referred to as YAG:Ce. Gadolinium may also be incorporated into the YAG structure to slightly shift the color of the emitted light (Gd-YAG:Ce). The addition of a cerium activator to the ceramic provides the means for light conversion. The cerium partially absorbs the blue light (wavelength of about 420-490 nm) emitted by the LED and re-emits yellow light with a broad peak around 570 nm. The mixture of the blue and yellow light renders the desired white light.
Color uniformity is an important aspect for the white light output by a pc-LED. For example, in automotive headlamp applications, the color uniformity of the beam projected on the road is important so that the headlamp is in compliance with SAE and ECE requirements. One key factor in projecting a beam of uniform color is that the LED package outputs light that shows minimal shift in color as the viewing angle varies around the LED. This however is not a trivial problem that is easily overcome.
The color of the light emitted by the pc-LED depends on the ratio of the amounts of unabsorbed blue light and converted yellow light which is affected by the path length the light travels within the converter. In particular, when the light emitted from the underlying blue LED travels through the ceramic converter, the light rays traveling normal to the chip surface have a shorter path to the light emitting surface of the converter than the light rays travelling through the ceramic converter at angles farther from normal. The amount of absorption (and subsequent re-emission at longer wavelength) follows the Beer-Lambert Law which shows an exponential dependence on both concentration and thickness:I/Io=10−εct  (1)where Io and I are the intensities of the incident and transmitted light, ε is the molar absorptivity of the absorber, c is the concentration of the absorber, and t is optical path length through the material.
Consequently, blue light travelling through the ceramic converter at angles farther from normal will be absorbed more strongly because of the longer optical path length in the material. This results is less blue light and more yellow light exiting the converter at larger angles thereby producing an overall emission that has a greater proportion of yellow light than the light emitted normal to the surface of the converter.
One solution to reduce the difference in angular color shift is to create a longer optical path for all light rays within the converter by introducing scattering sites in the form of pores in the ceramic material. Most ceramics are made by sintering a formed compaction of powders which contain a certain amount and size distribution of void space, referred to as “pores,” between the powder particles. These pores formed by the inter-particle spacing in the ceramic body are commonly referred to as matrix pores. The sintering process essentially brings the centers of the powder particles closer together, removes the porosity to some extent, and grows the grain size of the crystals in the ceramic material. Rather than trying to eliminate the porosity, the sintering temperature or sintering time may lowered so that the matrix pores are not all eliminated during the densification of the ceramic.
One drawback of using pore scattering in reducing the angular color shift is the reduction in efficacy associated with the excessive scattering by the pores. The effectiveness of the scattering will be determined by both the concentration and size of the pores in the ceramic. If the concentration of the pores is too high, the light will be substantially absorbed by the internal scattering and the overall LED output will be reduced.
The effect of pore size on the efficacy is reported as optimal with pores of diameter of about 800 nm in International Patent Application No. WO 2007/107917. The efficacy drops off rapidly with pore sizes below 500 nm and steadily for pore sizes above 1000 nm. However, it is difficult to control the size or the size distribution of the porosity through manipulation of the sintering cycle since too many factors, e.g., grain size, particle packing, grain growth, and sintering temperature, all influence the final pore population in the sintered ceramic converter. Thus, due to the thermodynamic and kinetic aspects of the ceramic processing, it is difficult to deliver a ceramic with the desired pore size and distribution.